The present invention generally concerns a variable speed drive for use with a screw conveyor, and more particularly concerns a combination of a mechanical adjustable variable speed belt drive with a torque-arm speed reducer for powering at user-selected variable speeds the drive shaft of a screw conveyor which is associated with the output of the speed reducer.
One long-term problem in the art specifically dealing with screw conveyor drives is to provide a practical and efficient variable speed drive. Fixed speed drives for screw conveyors are well known and widely used. One example of such is the Dodge-brand screw conveyor drive, a product of Reliance Electric Industrial Company, of Greenville, S.C., the Assignee of the subject application. Such screw conveyor drive includes a torque-arm speed reducer having a CEMA standard screw conveyor flange output and an output bore for drivingly receiving the screw conveyor drive shaft therein. CEMA (Conveyor Equipment Manufacturers Association) is a national organization which sets standards for screw conveyor components. Drive power is obtained from a fixed speed electric motor which is coupled to the speed reducer input through a belt drive.
While such Dodge-brand screw conveyor drive is a highly practical and reliable drive system for fixed speed screw conveyor installations, variable speed drive is not provided. Frequently, a screw conveyor is being used to feed materials of some type at a selected rate into a process of some kind. For whatever reason, the screw conveyor user may desire to adjust the material feed rate which would be accomplished by making a speed change of the screw conveyor drive shaft. Such change may be intended on a more permanent basis (as in a change to the process) or it may constitute simply an adjustment (fine or gross) to feed factors in an ongoing process. One approach could be to attempt to replace the fixed speed electric motor of the screw conveyor drive just described with a variable speed electric motor.
In general, while it is known to provide an electric motor with an electronic control system which would permit user-selected variation of the output speed of the electric motor, there are a variety of drawbacks associated with such an approach to this particular problem of screw conveyor drives. For example, electronically variable motor control systems require relatively higher designed-in power as a safety margin to prevent overheating. Screw conveyor drives have relatively higher torque requirements, which is another design factor which would tend to drive upwardly the power rating of any minimally adequate electronic control system. With higher power requirements comes higher current handling; with higher current handling comes higher heat generation, which will again dictate an accommodation thereof through increased motor size or other specialized heat sink features.
Another relative disadvantage with electronic motor control systems is the resulting increased physical size of the electric motor (due to the above-described design constraints) relative the replaced fixed speed electric motor, which can be a problem in some mounting situations. Also, any additional size and bulk presents design problems in mounting and support so as to prevent damage to the screw conveyor or the like due to undesired forces.
As in any system design, increased sizes and higher system capacity ratings invariably result in higher costs, which is generally undesirable.
The speed range capabilities of an electronic motor control system is also a factor. For example, a ten-to-one speed range is much more readily accomplished through a mechanical adjustable variable speed drive than with an electronic control system. Another speed range-related factor is the fact that some electronic control systems have difficulty in achieving a stable, relatively low operating speed, whereas mechanical drives generally have no such difficulty. One low speed drive problem associated with electronic control systems is referred to as cogging, which can result in unsmooth operation of the overall drive or even worse problems (such as stalling, vibration fatigue failures, and the like). It is also relatively speaking generally more difficult to slave an electronic control system (as compared with a mechanical adjustable system) to various processes with which some systems may be used.
One prior art alternative to attempting to provide an adequate electronic motor control system in place of a fixed speed electric motor attached via a belt drive to a torque-arm speed reducer is to replace the electric motor entirely with a hydraulic device which is coupled through a mounting plate to a torque-arm speed reducer. One example of such a device is the Dodge HYDROIL-brand drive, another product of the present Assignee, which can be coupled through a hydraulic motor adapter or coupling plate to the input of a torque-arm speed reducer, the output of which may be outfitted with a CEMA standard screw conveyor flange mount.
In general, a hydraulic drive is capable of infinitely adjustable speed and torque. It also generally has superior range, low speed torque capabilities, and low speed operation to that of a comparably rated electric motor with an electronic variable speed control system. Some examples of hydraulic devices are disclosed in Eddy (U.S. Pat. No. 3,942,414) and Myers (U.S. Pat. No. 3,817,665), both of which are commonly assigned to the present Assignee. While itself having advantageous characteristics and excellent performance for certain tasks, as is well known, a hydraulic drive requires a flow of hydraulic fluid for operation. Hence, in order to power the hydraulic drive, a suitable hydraulic pumping system must be provided. In addition, the hydraulic pumping system must be adequately rated to match the required performance standards for the hydraulic device. In other words, the hydraulic pumping system must be capable of providing an adequate hydraulic output to power the hydraulic device over its desired range. Again, in general, associated system costs increase in direct relation to increases in required system performance capabilities.
Another factor for consideration with respect to hydraulic devices is their intended use. In general, it may be desirable to avoid unnecessary presence of hydraulic systems in certain work environments. For example, the food industry, which makes widespread use of screw conveyors, is one example of a work environment where certain systems (such as a hydraulic drive) are preferably avoided.
Nonetheless, with reference to the food industry as one particular example, it is often the case that a screw conveyor is involved in a food preparation process or the like for feeding a component to such process. It may be highly desirable in such process, if not in fact a virtual requirement in some instances, that fine adjustments (or even gross setting changes) must be made in the rate of the feeding of the component relative the overall process. For such reasons, the technical problem comes full circle to the long-term problem of providing a screw conveyor with a dependable and acceptable variable speed drive.
Mechanical adjustable variable speed drives of different types have been generally known for some time, but not used in conjunction with a system for driving screw conveyors. The present Assignee provides a full line of mechanical adjustable variable speed belt drives, such as the Reeves-brand "MOTO DRIVE" for example, a handwheel may be used to input changes in the spacing of paired variable speed discs which are coupled by a variable speed drive belt, as well known in the industry, resulting in user-selected mechanical adjustable variable speed. Typically, a fixed speed electric motor is used to power the input, with the output feeding a concentric, in-line gear reducer. The present Assignee also provides an automatic controller known as the "RAC" for electronically controlling the mechanical adjustment for obtaining variable output speeds of the belt drive. See, for example, Woyton (U.S. Pat. No. 4,770,065) which is commonly assigned to the present Assignee. Examples of commonly assigned United States patents which disclose other aspects of some typical prior art mechanical adjustable variable speed belt drives include Huff et al. (U.S. Pat. No. 4,425,102); Meredith (U.S. Pat. No. 4,411,590); Huff et al. (U.S. Pat. No. 4,384,863); and Zigler (U.S. Pat. No. 4,370,139).
The present Assignee also separately markets Dodge-brand "TORQUE-ARM" speed reducers of either straight bore or taper bushing construction. Examples of commonly assigned patents which relate to speed reducers include Strang (U.S. Pat. No. 3,590,652) and Chung (U.S. Pat. No. 3,398,597). Heretofore, such speed reducers have generally not been combined through an interface coupling adapter device or means with a mechanical adjustable variable speed belt drive, nor so combined for use in driving a screw conveyor.
SEW-EURODRIVE, Inc., of Lyman, S.C., has a product which it refers to as the "VARIGEAR"-brand variable speed unit with a "SNUGGLER"-brand reducer. SEW's sales literature refers to both such brand names as being federally registered trademarks. The "VARIGEAR" product comprises a variable speed belt drive which is integrally provided with a reducer such that the shaft about which turns the pair of variable speed output discs of the belt drive comprises the first gear stage of the reducer.
SEW-EURODRIVE, Inc., also similarly incorporates its "SNUGGLER"-brand reducer into its "VARIMOT"-brand variable speed unit, which is a dry traction disc drive, a totally different construction from a mechanical adjustable variable speed belt drive. In the drive industry, the term C-flow means that the motor is on the same side of the drive case as the input shaft, while the term Z-flow means that the motor is on the opposite side. The "VARIMOT" unit with an integral "SNUGGLER" reducer is configured more like a Z-flow drive power arrangement, which requires considerably more mounting space than a C-flow drive power arrangement. The physical layout of the construction also affects the forces at the interface between the load to be driven and the output of the reducer. Such considerations would generally render such a configuration inapplicable to screw conveyor usage, and, in fact, the "VARIMOT" and "VARIGEAR" variable speed units with an integral "SNUGGLER" reducer are not thought to be available originally equipped with a CEMA standard flange mount as would be required for any screw conveyor application.